| Carbon structural materials are widely applicated in ultra-high temperature environment fields such as deep space,deep earth,aerospace and so on.However,carbon structural materials are easily oxidized at high temperature,how to improve the anti-oxidant performance have become the focus of the research.Although the silicon boride based protective coating forms a layer of borosilicate glass shows well oxidation protection capability in a wide temperature range,the oxygen barrier structure of the coating changes dynamically.Si B6 will oxidize to formed boron oxide and silicon oxide in high temperature,due to the readily gasification of boron oxide,holes were formed in the coating,resulting in its destructive dynamic evolution,weakening the oxidation defense capability of the coating.In order to inhibit the destructive dynamic evolution of silicon boride-based coating,in this dissertation,high active powders such as Mo Si2,Ta Si2 and Si B6 were prepared by combustion synthesis to improve the sintering performance of the coatings,and then combined with the low-temperature hot pressing sintering technology of spark plasma sintering to improve the compactness of the coatings and enhance the structural oxygen resistance of the coatings.Secondly,excellent film-forming characteristics of Mo Si2 and Ta Si2 were used to modify Si B6coating,through the fusion of transition metal oxide and borosilicate glass,a multiphase glass layer was formed to inhibit the volatilization of boron oxide,improve the dynamic stability of the coating in the oxidation process and reduce the oxygen permeability of the coating.Furthermore,in order to eliminate the interference caused by carbon matrix oxidation,relying on the data analysis mode of“standard sample-coating sample-matrix sample”,a high oxygen resistance evaluation mode was proposed to comprehensively evaluate the oxygen resistance of the coating.In order to overcome weak exothermic of silicon boride,Si Bx compound was successfully prepared by combustion synthesis(CS)method with assist of high exothermic Ti+Si+Ti O2+Al“chemical furnace”system of preheating and igniting Si-B powder.Si Bx synthesized process and adiabatic combustion temperature shows that Si-B reaction after chemical furnace combustion synthesis completed,and the reaction temperature is 1746K.According to the analysis of Si-B binary phase diagram,through solid solution reaction of liquid silicon and?phase boride to produced Si Bx compound.Rietveld phase quantitative analysis showed that when the silicon boron ratio reached1:6,the main diffraction peak in the final product was Si B6,Mo Si2 and Ta Si2was prepared by CS,which provided raw materials for the preparation of coatings.The 1773K oxidation test results of Mo Si2-Si B6 ceramic coating showed that coating has good compatibility with graphite matrix.With the mass increasing of Mo Si2,the cumulative oxidation protection efficiency of Mo Si2-Si B6 coating increased first then decreased.The 60%Mo Si2-40%Si B6 coating sample showed the best oxygen resistance,and its oxidation protection efficiency increasing from 89.8%to 98.1%,improved by 9.2%,this indicated the excellent inert modification capability of Mo Si2.During oxidation process,a double glass layer was formed on the outer surface of the coating and the joint between the coating and carbon matrix,which caused the thickness of the outer glass layer of 60%Mo Si2-40%Si B6 coating reduced from 82?m to 31?m,showed the lowest inertia factor(0.94×10-6 g·cm-2·s-1)of 60%Mo Si2-40%Si B6.with the combined action of coating self structure and surface glass layer of 60%Mo Si2-40%Si B6 coating,the oxygen permeability of the coating decreased from 3.79%to1.39%,improved by 63.3%;the carbon loss rate gradually decreased from 2.52×10-6g·cm-2·s-1 to 0.12×10-6 g·cm-2·s-1,improved by 95%.The 1773K oxidation test results of Ta Si2-Si B6 ceramic coating showed that with the mass of Ta Si2 increased from 20%to 80%,the cumulative oxidation resistance efficiency of Ta Si2-Si B6 coating increased from 86.1%to 98.0%.The80%Ta Si2+20%Si B6 coating sample shows the best oxygen resistance,and its oxidation protection efficiency was improved by 13.8%.With the mass increasing of Ta Si2,the structure factor of Ta Si2-Si B6 coating decreased from 0.4%to 0.11%,improved by72.5%,indicating that Ta Si2 is contribute to improve the oxygen resistance of the coating structure;inertia factor decreased from 2.12×10-6 g·cm-2·s-1 to 0.72×10-6 g·cm-2·s-1,improved by 66%,indicating that Ta Si2 also contribute to improve the oxygen resistance and protection ability of glass layer.The micro morphology analysis of Ta Si2-Si B6 coating after oxidation shows that Ta-B-Si-O multiphase glass layer was formed on the outer surface of the coating and the second glass layer was formed at the joint between the coating and carbon matrix,so that the thickness of the outer glass layer of80%Ta Si2+20%Si B6 coating reduced from 16?m to 10?m,which showed the lowest inertia factor(0.72×10-6 g·cm-2·s-1).With the combined effort of coating self structure and the surface glass layer of 80%Ta Si2+20%Si B6,with the micropores and microcracks formed on the coating surface are gradually reduced,the oxygen resistance of the multiphase glass layer gradually better,and the oxygen permeability of the coating is reduced from 4.12%to 1.42%,which improved by 65.5%;the carbon loss rate gradually decreased from 0.80×10-6 g·cm-2·s-1 to 0.21×10-6 g·cm-2·s-1,improve by73.8%.In Mo Si2-Ta Si2-Si B6 high temperature oxidation resistant coating system,when the mass of Si B6 increased from 10%to 30%,the cumulative oxidation protection efficiency of Mo Si2-Ta Si2-Si B6 coating increased first and then decreases.20%Si B6+20%Ta Si2+60%Mo Si2 coating sample showed the best oxygen resistance,and its oxidation protection efficiency increased from 90.6%to 98.9%,improved by9.2%.In the oxygen barrier evaluation mode,the smaller structural factor is,the better inhibition effect of the coating itself on oxygen diffusion is,the fewer number of oxygen diffusion channels in the coating is,the smaller inert factor is,and the stronger inert protection effect of the composite glass layer on the coating is.During oxidation process,a double glass layer was formed on the outer surface of the coating and the joint between the coating and carbon matrix.In the 20%Si B6+20%Ta Si2+60%Mo Si2 coating sample,the thickness of the glass layer at the joint between the coating and carbon matrix reduced from 6?m to 5?m which showed the lowest inertia factor(1.68×10-6 g·cm-2·s-1).Therefore,in the inert oxidation stage,the inert protection capability of the20%Si B6+20%Ta Si2+60%Mo Si2 oxidation sample is the strongest,which also shows that the coating volume change is the smallest(185%),and the lowest volume change shows that the oxygen resistance of the composite glass layer is the best,which effectively reduced the oxygen permeability of the coating from 3.98%to 1.09%,improved by 72.6%;the carbon loss rate gradually reduced from 0.76×10-6 g·cm-2·s-1to 0.18×10-6 g·cm-2·s-1,improved by 76.3%.This dissertation focuses on the research of high-temperature oxidation resistance of Mo Si2 and Ta Si2 modified Si B6 coatings,the established oxidation resistance evaluation system provides a quantitative reference standard for the design and characterization of other coatings in this field,and also provides scientific data for the application of Si B6-Ta Si2-Mo Si2 coating in the field of high temperature. |
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